During the drilling operation, drilling mud is pumped at high pressure through the interior of a drill pipe to and out through the nozzles of the bit and back to the surface exterior the pipe via the annulus between the drill string and the borehole wall. The purpose of this hydraulic system is multifold, including, cleaning the workface at the bit and carrying the drill cuttings back to the surface, lubricating and cooling the drill bit, stabilizing the borehole that is formed to prevent its collapse and providing a source of power to downhole equipment.
From time to time, a leak might develop between the interior and the exterior of the drill pipe to create a "short circuit" which reduces the effectiveness of the drilling fluid in performing its above listed functions. If such a leak goes undetected and is allowed to persist over time, the flow of the drilling fluid, which is typically loaded with solids, will erode or wash away enough of the material of the drill pipe at the location of the leak as to weaken the pipe to the point of separation (twist off). Lost pipe in the bottom of the well prevents further drilling of the well until such time as the separated portion is retrieved or "fished" from the well. Fishing operations are time consuming and expensive and not always successful. If unsuccessful, the well must be abandoned and a new well or a sidetrack begun. Regardless of the fate of the fishing operation, separated pipe represents a significant financial loss.
Another detrimental event that may occur is a flow restriction or blockage which also interferes with the effectiveness of the drilling fluid in flushing cuttings from the well bore, cleaning the workface, lubricating and cooling the drill bit, and providing a power source. Furthermore, a total blockage has been known to cause the hydraulic pressure in the drill string to rapidly increase with eventual rupture of the drill string or the standpipe which feeds the drilling fluid to the drill string at the earth's surface.
Thus it can be seen that leaks or blockages in the system can have serious consequences so that there is a serious need for effectively characterizing and monitoring the hydraulic system to detect and provide early warning of a leak (washout) or a blockage to allow the driller to act before the leak grows or the pressure increases, under the influence of the high pressure mud, to the degree at which the integrity of the drilling tubulars is jeopardized. It would also be advantageous if such characterizing and monitoring of the hydraulic system of the drilling operation were able to provide corrections to other downhole measurements affected by the hydraulics and to provide indications of operating efficiency of the equipment dependent on the utilization of the power provided by the circulating drilling fluid. It will be understood that there is significant utility in any means available to monitor the state and efficiency of downhole drilling motors which are driven by the flow of the drilling fluids.